1. Field of the Invention
This invention relates to highly attrition resistant catalysts, catalyst precursors and catalyst supports and to processes for making and using them.
2. Background
The use of silica as a support for catalysts or as a binder for catalyst particles is well known. The silica provides strength and attrition resistance and it acts to disperse the catalyst particles.
U.S. Pat. No. 2,904,580 discloses a process for producing acrylonitrile from propylene in a fluid bed reactor using a catalyst consisting essentially of bismuth phosphomolybdate supported on silica. The catalyst precursors in solution were added to an aqueous solution of an aqueous colloidal silica sol containing 30 wt % silica. U.S. Pat. No. 3,425,958 discloses a process for improving the flow properties of particulate silica (average particle size about 1 to 100 .mu.m) by bringing the particulate silica into contact with a solution of silicic acid and removing the solvent from the silica. When a catalyst is to be deposited onto the particulate silica, the catalyst precursor and any catalyst additive required are added to the silicic acid solution along with the particulate silica and the solvent is then removed. Heating is disclosed as a convenient way to remove the solvent. U.S. Pat. No. 3,772,212 discloses a fluidized bed catalyst for production of aromatic nitriles, the catalyst comprises a vanadium oxide, a chromium oxide, and a boron oxide as catalyst components and silica as a carrier and is prepared by spray-drying a silica sol containing vanadium, chromium and boron compounds. An aqueous 30 wt % colloidal silica sol was used. U.S. Pat. No. 3,746,657 discloses a process for making a fluidized bed catalyst comprising an oxide of molybdenum and a supporting material derived from a colloidal sol of an oxide of silicon, aluminum, titanium or zirconium, said process comprising preparing a slurry of the catalyst components and spray drying the slurry. The silica sol employed is preferably a low alkali aqueous silica sol containing 30 to 50 wt % SiO.sub.2. U.S. Pat. No. 3,044,965 discloses a process for making a fluidized bed catalyst consisting of bismuth silico-molybdate or bismuth silico-phospho-molybdate comprising forming a slurry of the appropriate metal compounds and silica and spray drying. A low alkali aqueous silica sol containing 30 wt % SiO.sub.2 was used as the silica source. U.S. Pat. No. 4,014,927 discloses a process for the production of unsaturated acids by catalytic oxidation of the corresponding unsaturated aldehydes in the presence of molybdenum-vanadium-iron based catalysts prepared by forming a solution or slurry of source compounds of the above metals and a source of silicon such as a silicate, water glass, or colloidal silica, and then drying by either evaporation or spray drying. U.S. Pat. No. 4,092,354 discloses a process for producing acrylic acid by the gas phase oxidation of acrolein comprising contacting acrolein and molecular oxygen over a metal oxide catalyst containing Mo, V, Cu, and at least one of Fe, Co, Ni, and Mg. The catalyst carrier is chosen from a group of materials, including silica sol and silica gel. Catalyst precursors in solution are added to a silica sol containing 20 wt % SiO.sub.2. The mixture is evaporated and then calcined.
U.S. Pat. No. 3,313,737 discloses a process for preparing an improved silica acid sol for use as a binder. The silicic acid sol is prepared from an alkyl silicate which is hydrolyzed by water in the presence of a mutual solvent by the catalytic action of a strong acid. These sols contain 10-30 wt % SiO.sub.2. U.S. Pat. No. 3,920,578 discloses rapid-gelling binder vehicles produced by admixing a water-soluble, alkaline ionic silicate with a colloidal amorphous silica aquasol having a median particle diameter of 50 A to 0.5 .mu.m. U.S. Pat. No. 3,894,964 discloses production of shaped bodies of zeolites of improved mechanical resistance (i.e., compression strength) using a silicic acid gel as a binder. The unstable silicic acid sol that is used has a silicic acid content greater than 10 wt %, usually 25-35 wt %, and a silicic acid surface area greater than 150 m.sup.2 /g. U.S. Pat. No. 3,296,151 discloses a process for the production of substantially spherical, silica bonded, zeolitic molecular sieve granules comprising forming a suspension by adding powdery molecular sieve zeolite to an aqueous silica sol, said silica sol having a surface area of 150-400 m.sup.2 /g on drying and said silica sol used in a concentration of 10 to 40 wt % SiO.sub.2 ; forming a suspension of finely divided magnesium oxide; admixing the two suspensions to produce a product having 0.1-3 wt % MgO; introducing the mixed suspensions dropwise into a liquid which is immiscible with water, whereby spherical granules are formed by sol-gel conversion; and separating the granules from the liquid and drying the granules. U.S. Pat. No. 3,356,450 discloses a process for preparing substantially pure zeolite granules starting with zeolite particles bound with silicic acid. The silica sols used to make the starting particles have surface areas of 150-400 m.sup.2 /g and at least 10 wt % SiO.sub.2, and usually of the order of about 25 to about 35 wt % SiO.sub.2. British Patent Specification No. 974,644 discloses a process for the production of molecular sieve pellets bonded with silicic acid, which comprises forming a plastic composition from a molecular sieve zeolite and an aqueous silica sol having a specific surface area between 150 and 400 m.sup.2 /g and a SiO.sub.2 concentration between 10 and 40% by weight. Preferably, the silica sol is produced by ion exchange of sodium silicate and subsequent thermal treatment at a pH of 9 to 10. The patent further discloses that it has also been proposed to use, as binding agents, silicic acid esters which are hydrolysed to silica gel by the water which is added to the mixture. The gels formed by the hydrolysis of the esters have a specific surface area of about 800 m.sup.2 /g and consequently consist of extremely fine particles. The use of the silicic acid esters is said to be too expensive for practical use, but experiments to replace them by normal commercial stable aqueous silica sols with a specific surface area between 100 and 200 m.sup.2 /g failed because the resulting granules had insufficient bonding strength and disintegrated.
U.S. Pat. No. 4,112,032 discloses a process for making porous silica-containing articles having pore diameters ranging between about 100 A and 1 .mu.m by combining a silicate solution containing at least 20% SiO.sub.2 and a colloidal silica solution containing 40 wt % SiO.sub.2 and then adding an organic gelation agent. Particulate matter less than about 74 .mu.m in diameter and selected from the group consisting essentially of alumina, titania, silica, zirconia, carbon, silicon carbide, silicon nitride, iron oxides, and catalytically active transition metal oxides can be added before the gelation agent to produce porous silica articles containg a powder phase dispersed therein. U.S. Pat. No. 3,629,148 discloses a process for making an attriton resistant iron-containing catalyst from a bismuth phosphomolybdate catalyst by forming a mixture of the prescribed ingredients in a silca dispersion and spray drying. The silica dispersion contained 30 wt % SiO.sub.2. U.S. Pat. No. 4,453,006 discloses a two-step process for preparing attrition resistant supported solid oxidation catalysts containing any known elements, preferably those containing molybdenum and used for the vapor phase oxidation of propylene or isobutylene to prepare unsaturated aldehydes and acids. The process comprises adding fumed silica to a mixture containing one or more active ingredients of the catalyst, drying said mixture, adding to this mixture in solution a silica or silica-containing compound other than fumed silica, and drying and calcining the mixture. The amount of fused silica can be 5-95% of the total silica used, with 15-65% being preferred. The silica used in the second addition can be silica sol, silica gel, diatomaceous earth or any precursor to silica, such as silicate, that preferably has a surface area of 50 m.sup.2 /g or more. The silica sols used in the Examples have silica contents of 40 wt %. U.S. Pat. No. 4,400,306 discloses a process for preparing supported attrition-resistant catalysts for fluid bed reactors by impregnating a preformed support, e.g., silica, but also alumina, alumina-silica, zirconia, and niobia, with a metal alkoxide of at least one metal selected from vanadium, molybdenum, antimony, copper, niobium, tantalum, zinc, zirconium, boron and mixtures thereof, and contacting the impregnated support with a solution of at least one additional catalyst component in situ, and drying the catalyst-containing support.
The preparation of mixed oxide compositions of vanadium and phosphorus and the use of these as catalysts for the oxidation of hydrocarbons such as n-butane to maleic anhydride is known in the art. In U.S. Pat. No. 4,111,963 the importance of reducing the vanadium used in a vanadium/phosphorus oxide (V/P/O) catalyst to the +4 oxidation state is described. Preferred is the use of concentrated hydrochloric acid as the reaction medium to bring about this reduction and preferred catalysts have a phosphorus to vanadium atom ratio of 1:2 to 2:1 and a porosity of at least 35%. In U.S. Pat. No. 3,864,280 the reduction of the vanadium in such a catalyst system to an average valence state of 3.9 to 4.6 is emphasized; the atomic ratio of phosphorus to vanadium is 0.9-1.8:1. Isobutyl alcohol is used as a solvent for the catalyst preparation, with the indication that an increase in catalyst surface area, over that obtained from use of an aqueous system, is achieved. The addition of promoters to the vanadium/phosphorus oxide catalyst compositions used for the oxidation of hydrocarbons to maleic anhydride is also disclosed in the art. Thus, in U.S. Pat. Nos. 4,062,873 and 4,064,070 are disclosed vanadium/phosphorus/silicon oxide catalyst compositions made in an organic medium. In U.S. Pat. Nos. 4,132,670 and 4,187,235 are disclosed processes for preparing high surface area vanadium/phosphorus oxide catalyst. Anhydrous alcohols of 1-10 carbon atoms and 1 to 3 hydroxyl groups are used to reduce the vanadium to a valence of 4.0 to 4.6. Also disclosed, as in U.S. Pat. Nos. 4,371,702 and 4,442,226, are vanadium/phosphorus oxide catalysts containing the promoter comprising silicon and at least one of indium, antimony and tantalum, the Si/V atom ratio being in the range 0.02-3.0:1.0, the (In+Sb+Ta)/V atom ratio being in the range 0.005-0.2:1.0 and the P/V atom ratio being in the range 0.9-1.3:1.0, said catalyst being prepared in an aqueous or organic liquid medium by the procedure wherein the appropriate vanadium species substantially of valence +4 is contacted with the promoter or promoter precursors and thereafter with the appropriate phosphorus species.
The attrition resistance of the vanadium/phosphorus oxide catalyst is particularly important when the oxidation process is carried out in a fluid bed or recirculating solids reactor. U.S. Pat. Nos. 4,317,778, 4,351,773, and 4,374,043 disclose processes for preparing fluid bed vanadium/phosphorus oxide catalysts in which an aqueous slurry of comminuted catalyst precursor is spray dried. Preferably, the catalyst precursor is uncalcined when it is made into a slurry. Examples are given in which an aqueous slurry of the catalyst precursor and a silica sol is spray dried to provide the catalysts 80 wt % V/P/O -20 wt % SiO.sub.2 and 70 wt % V/P/O -30 wt % SiO.sub.2. The products are described as uniform, microspheroidal catalyst particles. U.S. Pat. No. 4,127,591 discloses a process for preparing fluid bed vanadium/phosphorus oxide catalysts containing potassium and iron in which an aqueous slurry of the catalyst precursor is spray dried. Examples are given in which an aqueous slurry of the catalyst precursors in solution and a silica sol, 20 wt % silica, is spray dried to provide the catalyst 65 wt % V/P/K/Fe/O -35 wt % SiO.sub.2. Silica content of the catalyst is to be between 25 and 70 wt %. British Patent Specification No. 1,285,075 discloses a process for preparing attrition-resistant vanadium/phosphorus oxide catalysts for fluid bed reactors by spray drying a mixture of a vanadium compound, a phosphorus compound, and an aqueous silica sol. The silica sols used in the Examples contained 30-35 wt % SiO.sub.2. British Patent Specification No. 2,118,060 discloses a process for preparing a catalyst comprising oxides of vanadium and phosphorus by mixing two crystalline oxides, each containing vanadium and phosphorus and each with a specified X-ray diffraction pattern, with a silica sol, spray drying the resultant slurry, and calcining the particles obtained. The silica sols used were 20-40% silica sol solutions and 40% colloidal silica solutions. U.S. Pat. Nos. 4,062,873 and 4,064,070 disclose processes for preparing a catalyst comprising oxides of vanadium, phosphorus, and silicon by coprecipitating vanadium oxide and silica or a silica precursor. Phosphorus can be coprecipitated with the vanadium oxide and silica or silica precursor or added later to form the catalyst precursor, which is then calcined to give the silica-containing catalyst. The catalysts of the Examples contain 0.7-5.3 wt % silica which is distributed uniformly throughout the pellet. Russian Patent No. 215,882 discloses a method for preparing a vanadium/phosphorus oxide catalyst which is said to have increased activity and increased mechanical strength. Industrial large-pore silica gel is impregnated with a heated solution of oxalic acid, phosphoric acid, and vanadium pentoxide, dried, and activated. U.S. Pat. No. 4,388,221 discloses a process for preparing vanadium/phosphorus oxide Sn-containing catalysts comprising mixing the catalyst precursor, a binder, solvent and mordenite to form an impregnated mordenite which is then calcined. Silica is one of the suggested binders and the binder is said to comprise 0 to 10 wt % of the finished composite catalyst.
The objective of this invention is to provide a method for making attrition resistant catalysts, catalyst precursors and catalyst supports.